Alloy steel



Patented Jan. 9, 1934 UNITED STATES PATENT OFFICE No Drawing.Application December 3, 1928,

Serial No. 323,572, and in Germany January 19,

Alloys of iron and aluminum in themselves are known.. Their use ishowever very limited, as although they have good properties, these areaccompanied by a greater number of unfavorable ones, depending upon theamount of the content in aluminum.

4% aluminum steel has been worked up into dynamo and transformer platesin ,order to utilize that property of aluminum which has a similareffect to silicon on the magnetic losses of soft iron. The use of thishas however been given up as the aluminum plates in other respects hadnot properties of the same value as the silicon sheets and were moreovermore expensive.

German Patent No, 431,314, class 18b describes alloys which it isclaimed possess the special advantages of a high electrical resistanceand great resistance-to heat. They contain from,8 to 16% of aluminum, 6to 18% of chromium or 6 to 8% of manganese. These alloys belong to theclass of high heat resisting alloys, which in general show a highpercentage content of chromium and nickel, with Or without otherconstituents, as for example, manganese, cobalt and in a few cases alsoaluminum between 8 and 60%.

Apart from theirhigh price these high alloy steels cannot be used formany purposes, for instance boiler tubes, on account of their Martensitestructure in the rolling or forging state, and the difiiculty of workingthem. I

Separate technological tests, as for example the hard bending test forsteam tubes (according to the resolutions of the German Steam Boiler 0Commission on regulations with regard to material and work forstationary boilers, of June18, 1926), as also the rolling, etc. of thebeforementioned high alloy materials, meet with almost insuperabledifiiculties, apart from the difficult and almost impossible productionof tubes.

High alloys of iron-aluminum, having content of more than 10% aluminum,and which are heat resisting up to more than 900 C., are also known, butthese alloys are only used for castings, as they cannot be rolled orforged In several prior patent specifications ironaluminum alloys aredescribed, all of which have high aluminum content (never under 5%) andhigh nickel and chromium contents. For example:

2 Claims. (01. 75-1 United States patent specification 1,633,805

Per cent Aluminum .10 to Chromium 5 Nickel United States patentspecification, 1,633,826

I Per cent Aluminum 8 to 15 Nickel 25 Chromium"--. 7 to 15 Britishpatent specification 220,006 Up to 40% chromium with high manganese,nickel and tungsten contents. The addition of cobalt, copper andaluminum in small quan: titles is only incidentally mentioned as alloycon stituents. British patent-specification 215,231 I Per cent Chromium-1 15 to 35 Aluminum 5 to 12 begins to scale in an oxidizing atmosphereas low as 600 C3 A soft low percentage alloy which would not showscalingat 700to 800? 0. would be an advance, as these temperatures are for manyproductions partlytreatment and partly working temperatures.

The present invention is based on the results of exhaustiveinvestigations carefully carried out over a long period in thisdirection, and which have led to a material which satisfies for examplethe requirements of modern high capacity steam boilers. The intendedincrease of steam temperature to about 600 C. has hitherto been pre-.

vented for the want of suitable material, which not only had to be scaleresisting but whichcould be rolled without difficulty and which inevery.

other respect would satisfy the various treatment and workingrequirements of the above mentioned technical tes s. The material of theinvention has prppertie which the high alloy, high temperature resistingsteels, apart from their high cost, did not possess.

The essence of the invention lies in the recognition that 2.7% ofaluminum is sufiicient as an alloy to make ordinary steelscale-resisting up to about 800 C., and that aluminum content as high asthat made known by the before-mentioned proposed alloys, must beregarded for many purposes as disturbing or even as actuallyprejudicial. The economical progress shown in the price alone, will beeasily recognized.

In contradistinction to the high alloy, high heat resisting steelsythislow alloy aluminumsteel may be subjected without difliculty and withoutspecial precautionary measures to the most diverse punching and rollingprocesses (such as the Ehrhardt, Stiefel, Mannesmann etc.) and may forexample be rolled into tubes.

Thus the invention relates to a steel alloy which is used in the rolled,drawn, punched or forged state and which is scale-proof up to 800 C.,said steel comprising a low alloyed soft steel containing about 2.7 to4%, or preferably 3 to 4% of aluminum. The balance of the alloy consistsprincipally of iron in addition to the usual nonmetallic constituents,which preferably are present in the relatively low amounts ashereinafter specified. Furthermore, it may contain, as will be seen morein detail from the following specification, such elements, which inrespect to the solubility of the alpha-iron for the carbon, operate in acontrary manner to aluminum in a total amount up to about 2%. Eachconstituent of said type may be present in amounts lower than 1%.Silicon may be used in small amounts. Other alloying constituents arenot intended to be present in the alloy in appreciable amounts whichwould influence the character ofthe alloy.

A further advance is the discovery that this steel, which is in itselfcoarse grained, may by suitable heat treatment be improved,-that is,made tough and ductile. By heating, with subsequent quenching, fromtemperatures above 700 C. up to about 1000 C. and re-heating to 500 upto 750 C. a tough steel may be obtained. The technical tests on boilertubes are more easily met and the steel may be drawn out for tubes ofsmaller diameters. It should be here mentioned, that unlike the usualheating, the first heating in the region of the solid solution shouldpreferably last longer, in order to ensure the complete diffusion of thecarbon in the interior of the aluminum-ferrite. As above a certain limitof concentration, which lies below about 2% aluminum, the gamma (7)phase is entirely suppressed and according to experiments made by theKaiser-Wilhelm-Institute for the investigation of iron, only one holdingpoint'(Ac at 735 Ar at 732 C.) could be detected. It is not necessary inthe improving process to quench the steel above 800 C.; as, with contentabove 2% aluminum, it is impossible to reach the field of the solidsolution, but through the quenching abetter distribution of the pearliteis obtained and besides even with longer subjection to heat, scaling isprevented. A further advantage of the steel produced by the new processis, that with it, such processes of treatment as hitherto necessitatedspecial intermediate treatment for the which do not need to be speciallytough, the following alloy is preferably chosen:

These low alloy aluminum steels may be rolled into plates, bars-orshapes and made into tubes of large and small dimensions withoutdiificulty, by the most diverse punching and rolling processes. They arescale-resisting up to 800 C., and are suitable, for instance, for gasgenerator retorts, for tubes in which a gaseous or fluid medium is to beheated, and for other purposes. With a tensile strength of about 40 to50 kgs/mm in the rolled state, the elongation was from 10 to 18%. Theyare easily electrically welded and are worked without difllculty.

2. A second way of carrying out the process is specially suitable forarticles which should be tougher. A steel article produced according tothe previously described process is subjected in the rolled state to anannealing or improving process, in which it is for example heated at 750simply heating at 600 to 700 C., which on account of the low cost, ispreferable for objects not needing great ductility.

3. By means of another way of carrying out the process, a finerstructure, and above all, more evenly distributed carbon contents, areat once obtained. For this, certain additions are re- .quired as alloyelements, which in respect of the solvent properties of the alpha (0:)iron for the carbon, act in a contrary manner to aluminum.

Exhaustive experiments have shown, that with quite small additions 'ofchromium or manganese, or of both together, a finer grained structuremay be obtained in the cast metal and consequently in the forged orannealed state also. Tungsten, molybdenum, vanadium, titanium etc. aloneor jointly, act more strongly in this direction. These elements also acton the tensile properties in known manner. One or more elements shouldtherefore be chosen, which, besides, producing a finer grain, also givethe desired tensile strength. For instance the aluminum steel is alloyedwith molybdenum or vanadium, or with both together, if a high range ofheat resistance is required. Titanium is to be preferred for im-'parting shearing strength.

The forged state structure of this steel is considerably finer grainedand more even, than without these additions. The alloy elements aregiven in each case.

4, In a further manner of carrying out the process, where it is desiredto obtain great ductility, these steels are quenched down from about 800C. and re-heated at 500 to 750 C., as mentioned earlier. A certaindegree of toughness may be obtained by simply heating up these steels at600 to 700 0., instead of quenching and tempering them.

The following tables show a few values, which clearly indicate theimprovement in the toughness depending on the alloy additions and theimproving treatment. The high elastic limit as compared with the tensilestrength is worthyof note, as it often reaches 80 per cent.

a e a a P n i ti Hand 5 421E133 g g g g g ,1 Treatment, degrees C.

Efi iii gas a 8 Q E Q Alloued with 36. 4 46. 5 15, 0 Annealed at 940.326% 35.4 44. 2 30. 0 Qiggi ched from 940, tempered a4 6 44. 2 28.4Quenched from 800, tempered.

650. 2.7% aluminum 0 48. 2 21. 4 As delivered. 0.8% chrom1um 44. 7 27. 6Quenched from 940, tempered 650. 2.8% aluminum. 0.8% chromium. 39.452.5163 As delivered. 0.1% molybdenum. 35.8 51.828.8 Quenched from 940,tempered 0.1% tungsten 650.

47.5 59.4142 As delivered. 35. 4 54. 8 l8. 3 Annealed 940. 44.1 56.1250Quenched from 940, tempered 36% aluminum 0.7% chromium... 39.7 54.024 2Quenched from 800", tempered 3% molybdenum 650.

38. 7 52.2 29 2 Qugggzhed from 940, tempered 7 48. 8 59. 9 20. 4Annealed at 680.

Below are further explanations with regard to the tables referring tothe new process and the new material:

The following remarks apply to other structures of such steels:

A fine grained micro-structure after quenching, similar to austenite, ofa steel with 3% aluminum and 0.3% tungsten is extraordinarily brittle inthis condition. The carbon is completely in solution; the fracture iscoarse crystalline. After tempering this quenched steel the carbon inthe separate austenite fields separates out as cementite, the lamella:of which are rather widely separated, so that a considerably highercarbon content, with more active etching, is shown. The elongation ofthe test bars (1=5d), which was 1.2% in the quenched state, has risen to30.5%; and the contractidn from 6.2 to 70.8%.

The object of the inventionis fdrther to produce material according tothe various forms of carrying out the new process, suitable for makingarticles capable of resisting scale formation up to 800 0., especiallytubes, the material consisting of low alloyed soft steel containingabout 2.7% to 4% aluminum. In particular the invention comprises the useof this new material, utilizing its special properties, in particular asmaterial for the making of such objects as may be produced by knownrolling or drawing processes, as for the production of drawn or rolledtubes. Finally a part of the invention is the means to produce inparticular superheater tubes, steam boiler and stay tubes, as well asother steam boiler components, especially for high pressure boilers,from a low alloy soft steel containing about 2.7 to 4% aluminum.

What I claim and desire to secure by Letters Patent of the United Statesis:

l. A rolled, .drawn, punched or forged steel material, scale proof up to800 C., which is made of a low alloyed soft steel containing about 2.8%of aluminum, about 0.8% of chromium, about 0.1% of molybdenum. and about0.1% of tungsten, while the balance consists of iron besides theusualnon-metallic constituents and is free from further alloyingconstituents.

2. A rolled, drawn, punched or forged steel material, scale proof up to800 C., which is made of a low alloyed soft steel containing from about2.7% to about 3.6% of aluminum, from about 0.7% to about 0.9% ofchromium, from about 0.1% to about 0.5% of molybdenum, and from about0.1% to'0.5% of tungsten, while the bal-

